The embodiment of the disclosure is generally related to optical power measurement in a passive optical network and, more specifically to a technique that allows optical power measurement to be performed under burst-mode conditions over a wide range of power levels by accumulating the measurement over multiple upstream bursts in a passive optical network.
A Passive Optical Network (PON) consists of an Optical Line Terminator (OLT), which resides in a Central Office (CO). The optical line terminator services a number of Optical Network Units (ONUs) which reside at a premise of an end user of the service. The ONUs are typically connected to the PON in a star arrangement using passive optical splitters. The upstream data on the PON going from the ONUs to the OLT is time division multiplexed. Since each optical network unit may be located at a different distance from the optical line terminator, the amplitude of the upstream signal, seen at the optical line terminator, varies between optical network units. The ability to accurately and non-intrusively measure upstream power for each optical network unit has many applications such as monitoring ONU laser wear over time, detection of rogue ONUs and optical power leveling between the ONUs.
Two key benefits provided by the embodiment of the disclosure are increased measurement accuracy when the input optical power is low in addition to solving the problem of increased latency associated with low input power. For a typical prior art a very long duration upstream burst would need to be scheduled to make an accurate measurement, since the prior art involves use of a log amp and the nature of the log amp results in slow response at low input levels. Log amps are typically used to measure signal strength since they have an output function that is the log of the input function. The log amp response time depends on input signal level; therefore at low input signals the response can be very slow. The ITU standard for GPON (G.984) only allows for a maximum upstream burst size of 125 microseconds. This in effect places a hard lower limit on the optical power level that can be measured with conventional continuous-mode RSSI circuitry. This hard lower limit does not cover the full range of optical power levels at which the system is expected to operate. A further complication is that scheduling extremely long upstream bursts to accommodate conventional RSSI circuits is very disruptive to the quality of service requirements for the upstream data traffic and wastes bandwidth. With the multi-burst approach of the embodiment of the disclosure the measurement is accumulated over several small naturally occurring upstream cells, thus no special RSSI burst scheduling is required which might disrupt the normal traffic flows reducing bandwidth.
Some conventional solutions provide an output voltage that is linearly proportional to the average input photodiode current. These approaches have limited dynamic range since the resulting output voltage is on the order of several volts at the high end of the input optical power range and only a few millivolts at the low end of the input optical power range. If this output voltage is referenced to ground potential, great care is required in the circuit implementation to overcome effects such as noise and offset voltages. In addition, the Analog to Digital Converter (ADC) used to measure the voltage must have very high resolution in order to meet the accuracy requirement at low power levels while still being able to measure the voltage when the input optical power is high. High ADC resolution results in increased system cost as well as longer ADC conversion times which exacerbates the problem with measurement latency requiring even longer upstream bursts to be scheduled by the PON MAC. Logarithmic amplifiers can be employed to reduce the demand for high ADC resolution, but they substantially increase the measurement latency as explained in the previous paragraph.
Therefore, what is needed is a non-intrusive method for measuring optical input power for the purposes of monitoring optical system conditions and allowing the optical system equipment to adjust system parameters during normal operation in order to improve system performance. Implementing such a measurement methodology for new optical network technologies such as GPON (defined in ITU-T Recommendation G.984) requires a measurement technique with wide dynamic range and greater accuracy than is offered by existing solutions.